1. Field of the Invention
The invention relates to cutting structures for rotary drag-type drill bits, for use in drilling or coring holes in subsurface formations.
2. Description of Related Art
Rotary drag-type drill bit usually include a bit body having a shank for connection to a drill string, a plurality of cutting structures at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutters. Each cutting structure includes a preform cutting element comprising a front facing table of superhard material bonded to a less hard substrate. The cutting element may be mounted on a carrier, also of a material which is less hard than the superhard material, which is mounted on the body of the drill bit, for example, is secured within a socket on the bit body. Alternatively, the cutting element may be mounted directly on the bit body, for example the substrate may be of sufficient axial length that it may itself be secured within a socket on the bit body.
In drag-type drill bits of this kind the bit body may be machined from metal, usually steel, and sockets to receive the carriers or the cutting elements themselves are machined in the bit body. Alternatively, the bit body may be moulded from tungsten carbide matrix material using a powder metallurgy process. Drag-type drill bits of this kind are particularly suitable for drilling softer formations. However, when drilling soft, sticky shale formations in a water based mud environment, and in other similar conditions, there may be a tendency for the shavings or chips of formation gouged from the surface of the borehole not to separate from the surface and to be held down on the surface of the formation by the subsequent passage over the shaving or chip of other cutters and parts of the drill bit. Also, there may be a tendency for such material to adhere to the surface of the bit body, a phenomenon known as "bit balling", eventually resulting in the bit becoming ineffective for further drilling.
Attempts have been made to alleviate this problem by suitably shaping the bit body itself adjacent each cutting element, so that chips or shavings of material removed from the formation by the cutting element engage the shaped part of the bit body in such a manner as to tend to break the chip or shaving away from the surface of the formation. Arrangements of this type are disclosed, for example, in U.S. Pat. No. 5,582,258. The effectiveness of such arrangements depends on the accurate location, shape and orientation of the shaped part of the bit body in relation to the location, shape and orientation of the cutting element, and difficulties can arise in ensuring the required cooperative relationship between the cutting element and the shaped part.
For example, it is usually necessary or desirable that all shaping of the bit body is completed before the cutting elements are mounted on the bit body. This is particularly the case when the bit body is moulded using a powder metallurgy process. This means that all the shaped parts of the bit body must be formed before the cutting elements are mounted on it, and accordingly any subsequent tolerances in the subsequent location and orientation of a cutting element may result in it not being in the optimum relation to the shaped part of the bit body. Also, each design of bit body is only suitable for one arrangement of cutting elements, since the orientation of the cutting elements is determined by the shaped parts of the bit body. It is not possible to employ on the bit body cutting structures in which the cutting elements have orientations (such as back rake, side rake etc.) which differ from those for which the shaped parts of the bit body are suitable.